Partially stabilized zirconia (PSZ) powders are of interest because they are useful in the fabrication of high strength zirconia ceramics which have high fracture toughness and resistance to thermal shock, and are extremely wear resistant. Such materials, which have been described as "ceramic steel" and "toughened ceramics", are useable under extreme conditions, particularly at high temperatures and in the presence of aggressive chemicals, in numerous applications. They are used, e.g., in extrusion and drawing dies, in valve guides and seatings, in tool bits for metal shaping operations such as turning, milling and punching, and in fuel cells.
Zirconium oxide, or zirconia, is a refractory oxide known to exist in three crystalline forms which are interconvertible and which are stable over certain temperature ranges. Up to about 1170.degree. C., the monoclinic habit occurs, between about 1170.degree. C. and 2370.degree. C. the tetragonal habit forms, and from about 2370.degree. C. up to the melting point (about 2680.degree. C.) zirconia is in the cubic form.
Fully stabilized zirconia consists entirely of the cubic crystallite structure and remains in this form over a wide temperature range due to the presence of sufficient amounts of stabilizing oxides, such as, for example, magnesium oxide (magnesia), calcium oxide (calcia), yttrium oxide (yttria) and/or cerium oxide (ceria). These oxides may be present along with zirconia either individually or as mixtures.
In partially stabilized zirconia, the predominant crystalline forms are cubic or tetragonal, the proportions of each being determined by the method of preparation. When the predominant form is tetragonal, some monoclinic form may be present also, but very little cubic. When the zirconia in the PSZ is mainly in the cubic form, the other forms are present only in small amounts. The oxides that are effective as stabilizers in PSZ are the same ones as those used in fully stabilized zirconia, although lesser amounts are used. PSZ can be processed to high strength and toughened ceramics by utilizing the volume expansion and shear strain developed in the tetragonal to monoclinic transition.
There are numerous references in the literature which describe the preparation of PSZ powders and ceramic bodies made from such powders. Several processes have been developed for the preparation of PSZ and most of them employ batch-wise procedures.
A traditional procedure is to mix commercially available monoclinic zirconia powder with stabilizer powders or their precursors and heat the mixture for several hours at 2050.degree. to 2600.degree. F., as described by T. W. Smoot, U.S. Pat. No. 3,301,646 (1967) and K. Yamada et al., U.S. Pat. No. 4,344,904 (1982). Powders produced by this solid state diffusion process do not have a homogeneous distribution of stabilizer and they are often sintered to relatively large agglomerates and aggregates. Size reduction by milling for several hours is necessary to produce powders suitable for ceramic processing. Such milling and classification processes lead to contamination. Powders obtained by the procedures of U.S. Pat. No. 3,301,646 consist of particles in the size range of 0.2 to 12 microns with 90 to 95% falling between 0.5 and 3.5 microns in diameter. In U.S. Pat. No. 4,344,904 there is no specific limitation on particle size although a preference is expressed for particles having a diameter not larger than 10 microns.
R. C. Garvie et al., U.S. Pat. No. 4,279,655 (1981) wet mill mixtures of zirconia and stabilizer powders to obtain a mean particle size of about 0.7 microns. The powders are dried and calcined at 800.degree. to 1450.degree. C. for about 24 hours and again wet milled until the particle size is about 0.7 microns. Although wet milling produces a more homogeneous composition, the process usually leads to contamination.
More uniform distribution of the stabilizer and zirconia can be obtained by co-precipitation of salts of both zirconium and the stabilizer species followed by calcining to form the oxides. The high temperature required to produce the oxides tends to create agglomerates which require a grinding operation to break them down and this can result in contamination. Alternatively a precursor solution of zirconium and a stabilizer can be co-hydrolyzed to form a homogeneous gel. The moisture is removed by a drying process and the resulting powder is calcined and milled. Somewhat lower temperatures, 1000.degree. to 1200.degree. C., can be used to produce PSZ and the particle size is smaller than in the previously described processes. However, some milling is necessary to obtain a powder suitable for processing to ceramic bodies.
Hydrothermal processes, such as those described by E. P. Stambaugh et al., U.S. Pat. No. 4,619,817 (1986) may be used to make PSZ powders with controlled particle size. This procedure uses an aqueous solution of zirconyl nitrate or zirconyl chloride to which a complexing agent, such as EDTA may be added. The pH is brought to 9 or more and MgO, CaO or Y.sub.2 O.sub.3 are added singly or multiply. Hydrothermal treatment in an autoclave at 190.degree. to 350.degree. C. produces powders having a median particle size in the range of 0.05 to 3.6 micron, typically 0.2 to 0.5 microns.
Each of the prior art compositions and processes suffers from various deficiencies, and further improvements are desirable.